When some of the providers (like nova_aggregate) were moved over to
openstacklib, region support was missing. This fixes the base provider
code to pass region_name into openstacklib if region_name is set.
Change-Id: I6953c10ceab708402ce25a23d1f8d20f3288cbad
Closes-Bug: #1517220
Depends-On: I3345dac8bebd68f93290c1d45aa9a4d80bf3fb34